WordWeb Online

1. Having momentous consequences; of decisive importance
   "that fateful meeting of the U.N. when...it declared war on North Korea"
   - fatal
    
2. Ominously prophetic
   - foreboding, portentous
    
3. (of events) having extremely unfortunate or dire consequences; bringing ruin
   "a fateful error"
   - black, calamitous, disastrous, fatal
    
4. Controlled or decreed by fate; predetermined
   "a fateful series of events"
   - fatal

See also: decisive, fatefully, inevitable, prophetic, prophetical, unfortunate